(1) Field of the Invention
The present invention relates to an image forming apparatus which directly forms the image on recording medium such as recording paper etc., by causing developer particles to jump thereto and can be applied to a printer unit in digital copiers and facsimile machines as well as to digital printers, plotters, etc.
(2) Description of the Prior Art
In recent years, as the image forming means for outputting a visual image on recording medium such as recording paper etc., in response to an image signal, an image forming apparatus is disclosed in Japanese Patent Application Laid-Open Hei 6 No. 155,798, for example, in which developer particles, i.e., toner, are made to directly adhere to the recording medium to thereby form a toner image on it, directly.
Referring to FIG. 1, the image forming apparatus defined in Japanese Patent Application Laid-Open Hei 6 No. 155,798 will be described. This apparatus includes an image forming unit 51 having a toner supplying section 52 and a printing section 53. In this apparatus, toner 71 carried in toner supplying section 52 is selectively made to jump to and adhere to a sheet-like recording paper 55 as a recording medium. During this operation, the jumping of toner 71 is controlled in accordance with an image signal so that the toner can selectively adhere to recording paper 55 directly, forming a visual image.
Toner supplying section 52 is composed of a toner reservoir 70 for holding toner 71 as developer particles which are, for example, negatively charged, and a toner support 72 for supporting toner 71 using, for example, magnetic force. Toner support 72 is grounded and rotationally driven in the direction indicated by arrow E in the figure, with its surface speed set at 30 mm/sec, for example. Toner 71 is of a magnetic type having a mean particle diameter of 10 .mu.m, and is electrified with static charge of -4 .mu.C/g to -5, .mu.C/g by a well-known technique. Toner 71 is carried on the peripheral surface of toner support 72 with a mean thickness of about 80 .mu.m.
Printing section 53 as a part of image forming unit 51 is composed of an opposing electrode 75 made up of an aluminum pipe of, for example, 50 mm in diameter, and a control electrode 76 which is provided between opposing electrode 75 and a toner support 72. Opposing electrode 75 is arranged about 1 mm apart from the peripheral surface of toner support 72, has a high voltage, e.g., 2 kV applied from a d.c. power source 80, and is rotationally driven in the direction of arrow F in the figure, with its surface speed set at 30 mm/sec. Therefore, generated between opposing electrode 75 and toner support 72 is an electric field needed to cause toner 71 supported on toner support 72 to jump toward opposing electrode 75.
Control electrode 76 is disposed in parallel to a tangent plane of the surface of opposing electrode 75 and spreads two-dimensionally facing opposing electrode 75, and it has a structure which permits the toner to pass therethrough from toner support 72 to opposing electrode 75. The electric field formed between toner support 72 and opposing electrode 75 varies depending on the potential being applied to control electrode 76, so that the jumping of toner 71 from toner support 72 to opposing electrode 75 is controlled.
Control electrode 76 is arranged so that its distance from the peripheral surface of toner support 72 is set at 100 .mu.m, for example. Control electrode 76 is composed of a flexible print board (FPC) 76a of 50 .mu.m thick and annular electrodes 77 are composed of a copper foil of 20 .mu.m thick. Board 76a has gates 79having a diameter of 150 .mu.m for passage of toner 71. Around these gates 79are arranged the aforementioned annular electrodes 77. Each annular electrode 77 is electrically connected via a feeder line and high-voltage driver (neither is illustrated) to a control power source 81.
Annular electrodes 77 are applied with voltages from control power source 81, corresponding to the image signal. Detailedly, when toner 71 supported on toner support 72 is made to travel toward opposing electrode 75, control power source 81 applies a voltage, e.g., 200 V to annular electrodes 77, so that toner 71 can jump through gates 79 of annular electrodes 77 to the recording paper on the opposing electrode side. In contrast, if the toner need not be passed, the power source 81 applies -200 V to annular electrode 77 to prohibit the toner on toner support 72 from jumping toward opposing electrode 75. In this way, the application of voltages to annular electrodes 77 is performed in accordance with the image signal, so that it is possible to directly form a visual image corresponding to the image signal, on the recording paper 55, by selectively causing the toner to jump.
Here, the rotation of toner support 72, the rotation of opposing electrode 75, the application of voltage to control electrode 76 to prohibit passage of toner 71, and application of the high voltage to the opposing electrode are activated at almost the same time by a common trigger. The transfer time of toner 71 from toner support 72 to recording paper 55 is determined depending upon the amount of static charge on the toner, the distance, and the potential difference applied, between toner support 72 and opposing electrode 75, and in particular, depends on the intensity of the electric field. This time is about 250 .mu.sec, for example. The voltage application time to annular electrode 77 is set longer than the transfer time, specifically at about 300 .mu.sec. Thus, the toner is ensured to adhere to recording paper 55 on opposing electrode 75.
In the above image forming apparatus of the prior art, in order to form a single dot on recording paper 55, the time during which the voltage is applied to the control electrode for causing the toner to jump, needed to be longer than the time required for the toner to transfer from the toner support to the opposing electrode, i.e., 250, .mu.sec. Specifically, the voltage application time needed to be as long as 300 .mu.sec, for example, to ensure the toner jumped and reached the recording paper. This becomes an obstacle for increasing the recording rate of this technique. In this way, up to now, it is impossible to expect this method to achieve high speed recording because of this time restriction.
At a higher resolution of the image, the printing speed must be even lower. In order to enable fast recording in the prior art system, the transfer time of the toner, that is, the time required for the toner to jump across the distance from the toner support to opposing electrode side, should be shortened. If this can be done, the time taken for application of voltage to the control electrode, can be shortened, so that it naturally becomes possible to perform high speed processing.
Nevertheless, the reduction in transfer time of the toner, involves many problems. Specifically, to shorten the transfer time of the toner, the electric field formed between the toner support and the opposing electrode should be enhanced. In particular, when the toner support side is grounded, the voltage to be applied to the opposing electrode may and should be increased to create a stronger electric field. However, the enhancement of the voltage not only needs an increased number of electric parts but also requires a greater level of insulation against high voltage to deal with the problems such as leakage, etc.
Also considered can be the shortening of the jumping distance, so that the transfer time can be reduced. However, there is a limit to shortening the jumping distance because of the size of the toner, the thickness of the recording paper and the thickness of the control electrode. Thus, as referred to before, it is impossible to reduce the distance between the toner support and the opposing electrode less than about 1 mm.
Another solution considered can be a modification of the property of the toner itself. However, the modification is very difficult under the present technology. Even if it were possible, it is impossible for a printer image of the type represented by the image forming apparatus disclosed in Japanese Patent Application Laid-Open Hei 6 No. 155,798 to shorten the voltage application time to the control electrode for toner jumping, because of the restrictions due to the aforementioned problems.